KVM: arm64: ITS: return 1 on successful MSI injection

According to the KVM API documentation a successful MSI injection
should return a value > 0 on success.
Return possible errors in vgic_its_trigger_msi() and report a
successful injection back to userland, while also reporting the
case where the MSI could not be delivered due to the guest not
having the LPI mapped, for instance.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>